The present invention relates to an apparatus for forming a recording medium layer on a disk, and more particularly to an apparatus for simultaneously coating a uniform thickness and composition of magneto-optical recording medium film onto several magneto-optical disks.
It has been noted that a magneto-optical recording medium (such as a magneto-optical disk) has been used as a good recording medium for reading and writing digital information. Because the magneto-optical recording medium is rewritable and has advantages of high density and reliability, the magneto-optical recording medium has become a commercial product in the market.
In the processes of making a conventional magneto-optical disk, an optical disk substrate has to be provided first, and it is usually made of polycarbonate; then, the optical disk substrate is coated with an under-coat, such as a lamina of aluminum nitride, which is usually done by using an aluminum sputtering target as a sputtering source; then, a magneto-optical recording medium is evenly containing alloy as transition metal and rare earth elements. On the surface of the magneto-optical recording medium, a cover-coat is coated by using the same material and method as that of coating the under-coat. Finally, a reflecting lamina (such as aluminum) and a protective plastic layer are coatedin sequence onto the surface of the over-coat to completely the whole processes. Normally, the prepared product above is tested by means of a known method of dynamic read/write test to assure the quality thereof.
In the aforesaid processes of coating a magneto-optical recording medium film onto the disk substrate, one of the most important steps is the process of forming the magneto-optical recording medium layer on the disk substrate. It is found that the materials of the alloy of transition metal and rare earth elements are preferable for the recording medium film. In the conventional method, a single disk substrate is set opposite to a sputtering target containing rare earth and transition metal element alloy in order to coat a magneto-optical recording medium film onto the disk substrate. During the sputtering process, the scattering flux of the elements of the sputtering target in sputtering area is not uniform. As a result, the composition of the alloy layer coated on the substrate is not uniform. In order to maintain the uniformity of the composition and thickness of the magneto-optical recording medium film on the substrate, it is preferable to use only the center area of the sputtering target as an effective sputtering area. Under this restriction, the manufacturer must utilize a sputtering target with a larger diameter than the diameter of the substrate disk so as to cover the surface of the disk substrate within the effective sputtering area of the sputtering target.
According to the conventional method, the cost of making such a magneto-optical disk would be higher because that sputtering target contains more terbium and needs a larger size covering larger sputtering area to meet the actual requirements. Furthermore, since only one disk substrate is set opposite to one sputtering target, so only one optical substrate disk can be processed each time, and therefore it wastes much time and manpower.